Lithium secondary batteries have the advantages of high energy density, little self-discharge, excellent long-term reliability and the like, and are therefore broadly put to practical use as batteries for small-size electronic devices such as laptops and cellular phones. In recent years, high functionalization of electronic devices and utilization of lithium secondary batteries for electric cars have progressed, and the development of higher-performance lithium secondary batteries is demanded.
At present, carbon materials are common as negative electrode active materials for lithium secondary batteries, and various types of carbon materials are proposed in order to improve the battery performance.
For example, Patent Literature 1 discloses a lithium secondary battery using an amorphous carbon material as a negative electrode active material and lithium manganate as a positive electrode active material, wherein the particle surface of the amorphous carbon material is coated with an oxidized film. Patent Literature 1 discloses that since the oxidized film suppresses formation of an inactive film to intercalation/disintercalation of lithium, and does not cause the extreme degrading even at high temperatures, the cycle life characteristics of the lithium secondary battery can be improved.
Patent Literature 2 discloses a lithium secondary battery which has a positive electrode and a negative electrode to intercalate and disintercalate lithium ions, and an organic electrolyte solution having an electrolyte containing lithium ions and dissolved therein, and in which the positive electrode and the negative electrode are arranged with a separator interposed therebetween; wherein a carbon material constituting the negative electrode is obtained by subjecting a non-graphitizable carbon to a heat treatment in an inert atmosphere or in such an atmosphere that the non-graphitizable carbon is slightly oxidized, removing decomposed gases, and thereafter subjecting the resultant to a heat treatment under pressure, and the density (butanol method) of the carbon material is 1.6 to 1.8 g/cc. Patent Literature 2 discloses as follows: lithium ions are intercalated and disintercalated mainly by 6-membered layers of the carbon material; then, the lithium ions intercalated by the 6-membered layers have only a low reactivity with the electrolyte solution; therefore, the capacity reduction of the negative electrode due to side-reactions can be reduced, and the lithium secondary battery excellent in the high-temperature storage characteristics can be realized.